Spinodal decomposition type phase transformation in a multicomponent alloy system is described, for example, in U.S. Pat. No. 3,806,336 issued Apr. 23, 1974, U.S. Pat. No. 3,954,519 issued May 4, 1976 and U.S. Pat. No. 4,171,978 issued Oct. 23, 1979. As has been described therein, a certain binary and other metallic system has, in its composition diagram, a "limit of metastability" or "spinodal" which is thermodynamically defined as the locus of disappearance of the second derivative of the chemical free energy with respect to composition of the system. When a high-temperature composition, which is of homogeneous single-phase structure, of the alloy is brought within the spinodal in a low temperature range, it is transformed into a separated two-phase structure, the phase separation being called spinodal decomposition. The decomposed alloy has a periodic microstructure generally in the order of hundreds of angstroms and which consists of composition modulated two isomorphous phases in which one phase is in the form of a fine precipitate unformly distributed in another phase which forms the matrix. It is observed that if the first phase in such a microstructure is magnetic and the second is nonmagnetic, there results a single-domain structure whereby a highly retentive magnetic body can be obtained. In an Fe/Cr/Co alloy, such first phase (.alpha..sub.1) is constituted by a Fe/Co-rich ferromagnetic phase and the second phase (.alpha..sub.2) is constituted by a Cr-rich paramagnetic phase.
It has been noted that during the cooling process, the high temperature single phase .alpha. is decomposed at a certain temperature corresponding to the miscibility gap of the system into two isomorphous phases: .alpha..sub.1 and .alpha..sub.2 phases. Since .alpha..sub.1 phase is magnetic whereas .alpha..sub.2 phase is nonmagnetic and because of the ultrafine size (about 0.03 micron diameter) and the desirably elongated shape of each individual of .alpha..sub.1 phase precipitates which are uniformly dispersed surrounded by .alpha..sub.2 phase precipitates, the resulting structure forms what can be called the single-domain structure.
On the other hand, attempts to thermodynamically analyze and synthesize the equilibrium phase diagrams of .alpha.Fe-X solid solution by computer calculations have recently resulted in substantial development. It was shown by Hasebe et al (c.f. Japan Society of Metals 1977 Fall Conference Proceedings) that the miscibility gap and its spinodal in .alpha.Fe-X solid solution are not simple parabolic but are of abnormal shape, extending toward the Fe side and forming a sharp "horn" or a broad bump at the Curie temperature. It was further shown that the addition of cobalt raises excessively the chemical potential of the alloying element in the ferro-magnetic state and enlarges remarkedly the magnetic anomalies in the solubility curve as well as the miscibility gap, which are in substantial agreement with the conclusion drawn by the present inventors from experimental data.
Further experimentation by the present inventors has confirmed the presence of the "horn" of the miscibility gap in the phase diagram of the alloy system and has also revealed magnetic properties of alloys in the vicinity of the "horn" as reported by one of the present inventors et al (cf. Japan Society of Metals 1978 April Conference Proceedings). It has been particularly pointed out that the rectangularity of the magnetic hysteresis curve is improved as the composition comes closer to the "horn" from the chromium side.